The standard correlation between S/C clock and UTC does
not permit the timing of events down to millisecond accuracy.
Therefore, a special method has been implemented:

The central data processing unit of the focal instrumentation
(ZDE) runs in a software cycle which is started every
second by a pulse provided by the S/C clock.
This cycle can be paused by other sources of interrupts,
e.g., on the reception of telecommands or the warm start of the
computer performed on reception of the on/off-command RESET.
Upon the reception of the RESET-interrupt, the ZDE needs
532.8 ms to perform a couple of tasks necessary for resetting
some registers etc., then it enters an idle-loop
which is stopped on the next one-second interrupt.
The time difference between the RESET- and the SECOND-interrupt
is measured by a software counter, one count corresponds
to 0.0113774 ms.

Whenever the RESET on/off-command is sent (typically once
per week during the first ground contact on monday),
the time when the command ``leaves'' the antenna is recorded
at the ground station.
As a source for an additional time delay, the distance
between ground station and the satellite has to be taken
into account.
This information is delivered by GSOC together with the time
when the command has been sent.
The value of the idle-loop counter is sent to ground by a
special telemetry frame.
The information from both channels is combined for generating
the clock correlation.

The S/C clock exhibits a slight drift
which has been specified to be smaller than .
The S/C clock is reset about once per year in order to prevent
an overflow of the 32-bit counter.
Occasionally, this has also happened accidentally.

SASS adds an offset to the S/C clock in order to create
homogeneously increasing event-times.
These event times (not the S/C clock!) are the relevant times
for the users and are, for instance, contained in the PETs.

For most applications, it is sufficient to perform a linear
regression to the correlation data within a period between
two consecutive clock resets.
The transformation from event-time (EVT) to UTC for
an interval i is

with DRF is the clock-drift, OFS is the offset added by SASS,
and RES is the time of a clock-reset.
All the relevant numbers can be found in
Tab. 3.6.
Clock-period 3 has been split into two sub-intervals at July 1,
when a leap second was inserted.
Differences between values calculated by this method and
the true S/C clock conversion are always less than 10 ms.
The residuals can be obtained from the
Figures 3.17 a) through e).

period

date

RES [d]

DRF

OFS [s]

1

01.06.90

151.879749069

-8.863748E-8

0

2

26.01.91

25.386330848

-8.934155E-8

20606971.33

3a

12.02.92

42.353305347

-8.401761E-8

53608920.63

3b

01.07.92

42.353293327

-7.956741E-8

53608920.63

4

19.01.93

18.705978489

-7.181173E-8

100663296.50

Table 3.6:Spacecraft clock resets

Figure 3.17: ROSAT S/C clock correlation for different periods:
a) Period 1, b) Period 2, c) Period 3, d) Period 4,
and e) Period 5